This invention relates to a mechanically timed electronic ignition system for an internal combustion engine.
French Patent Application No. 8017811 of Aug. 13, 1980 discloses a ignition system of the kind in which there is used a variable reluctance transducer of known type which is mechanically operated so as to produce an output waveform having zero crossings coinciding with the desired instants of ignition sparks. A winding of the transducer is connected by a passive circuit to voltage comparator means which form a part of an integrated circuit. At cranking speeds (i.e. during engine starting) the output of the winding is compared with a fixed threshold and the output of the comparator means is used to determine both the commencement and termination of current flow in an ignition coil controlled by the integrated circuit.
At cranking speeds, however, the amplitude of the transducer waveform is not much greater than the threshold voltage and, although various anti-interference provisions are made within the integrated circuit, there can still be problems caused by audio-frequency disturbances of small amplitude resulting from mechanical vibration or from magnetic field pulsations caused by the starter motor. These disturbances can be of sufficient amplitude to cause spurious early sparks, resulting in abrupt stalling of the engine due to reverse torque, which may in turn lead to starter motor damage or merely make starting extremely difficult.